Automatic frequency control circuits



Sept 5, 1939. w. F. EWALD Er AL 2,171,649

AUTOMATIC FREQUENCY CONTROL CIRCUITS Filed Deo. 15, 19.17 2 sheets-sheet 1 I. AM/1;, 3 2119 1 T142 ,4. F. fsu/ AAM/0L. 4

3 2 E sa 21 j f5 5i E "QM A :j I

f Si 7 5 0R11/ f f 9 o y('an fri/0 43 C'am/enser C F:

Sept 5, 1939. 'w. F. EwALD ET'AL 2,171,649.

AUTOMATIC FREQUENCY CONTROL CIRCUITS Filed Dec. l5, 19347 2 Sheets-Sheet 2 MW ATTORNEY Patented Sept. 5, 1939 UNITED STATES PATENT OFFICE AUTOMATIC FREQUENCY CONTROL CIRCUITS tion of Germany Application December 15, 1937, Serial No. 179,882 In Germany November 13, 1936 5 Claims.

Arrangements are known in the earlier art which have the purpose to insure automatic correction of tunin-g of a receiver apparatus for radio telephony after coarse setting of the tuning elements to the frequency band, or channel, of the desired station has been effected by hand. To this end, recourse is had as a rule to differential circuit schemes comprising two intermediate frequency circuits which are detuned in relation to each other a certain amount, or whose potentials present a certain phase shift, or angle, to each other so that a differential potential will exist between them as long as the carrier intermediate frequency differs from the rated value. The rectified differential potential is used in these circuit organizations for the direct, or indirect, shift of the oscillator frequency, this being accomplished by the variation of tube capacities, inductances, or mechanical adjustment of a variable condenser. Now, all of these arrangements involve the drawback that for the production of the desired effect several tubes and several tuned circuits are additionally required. As a consequence, the scheme is restricted to receivers of a kind in which the manufacturing cost is immaterial.

According to the present invention such circuit elements (oscillation circuits, rectiiiers, tubes) as serve for automatic sharptuning, by the use of circuit changing or switch means, especially of automatic action, after sharp tuning has been accomplished, are used wholly or partly for reception itself (for iiltering, rectification, or amplification).

In the drawings:

Fig. 1 is a circuit diagram of a superheterodyne receiver embodying the invention,

Fig. 2 is a vector diagram of the discriminator network potentials,

Fig. 3 is a detail view of the oscillator frequency regulator mechanism,

Fig. 4 is a view of the frequency control mechanism and tuning knob, and

Fig. 5 is a circuit diagram of a modified form of the invention.

Referring to Fig. 1 the numeral I denotes the conventional IF part of a superheterodyne receiver; details of the construction are immaterial so far as this invention is concerned, and are,

therefore, not shown. The amplified IF energy is fed to the diode rectiiier 2 through the IF band filter 3. If the tubes and circuit elements serve for signal reception, switch S1 may be closed and switch S2 (both on the left) is opened. In this case, the additional circuit elements which serve solely for automatic correction of tuning are in- (Cl. Z50-20) operative. The AF potential across the load resistance of diode 2 is impressed through a condenser 5 and a volume control resistance 6 upon the grid of the AF amplifier tube 1. The end, or power, tube 8 is in resistance coupling relationship with tube 'I in the usual manner. The switch S5 in this case connects the grid leak of tube 8 with ground.

Now, in what follows it shall be described what switchings, or circuit changes, are required for the purpose of utilizing the band-pass filter just described and the AF amplifier for automatic sharp tuning, or tuning correction. In order that a control potential which a function of the de tuning may be made available for the regulator, or control device, later to be described in more detail, recourse may, for instance, be had to the fact that the potential in the secondary coil III presents a quadrature phase relation in reference to the potential at the primary coil 9 when the incoming IF has a Value to which the two circuits are tuned. The upper end of the coil 9, by clos.- ing of the switch S2, is united with the midpoint of coil IIB. The condenser 34 serves solely for isolating the plate potential from the secondary circuit. The potential prevailing at point II to ground is then equal to the difference of the potentials at coil 9 and the lower half of coil ID; the potential at point I2 is equal to the sum total of the above-mentioned potentials. The vector diagram in Fig. 2 will make it clear that the potentials are alike whenever the phase diierence is exactly 90 degrees; in other words, when the proper intermediate frequency, and thus accurate tuning, exist. As long as this is not the case, the potential at the coil I shifts and the potentials at points I I and I2 become dissimilar or unequal.

The potential at point I2 is rectified by the diode 2, which previously served for the production of the audio energy; and the potential at point II is rectified by an additional diode I3. The switch Si must then be opened, and the midpoint of coil I0 must be connected with point I5 through an RF choke coil I6 and a switch S3 in order that the diode rectier current may be able to iiow. Across the load resistances 4 and I'I of the two diodes arise the rectified potentials whose diiference is between I8 and ground. In the light of what has been pointed out above it will be readily seen that this differential potential disappears in the presence of proper tuning, and that, when there is deviation of the IF in one sense or the other, it assumes positive or negative values. Through a lter network I9, 20, and a switch S which at tuning must be closed at the left contact, the same is impressed, for instance, upon the grid of the power tube and is amplified in the latter. The -grid of the pre-amplifier (or input tube) 1 is short-circuited while the tuning proceeds, conveniently by the aid of the switch S4 in order that no additional AF may be fed to the power tube. When the switch Se is opened,Y

the plate circuit, in addition to the AF transform.er,'contains a coil 2l the magnetic Yeld `of which (varying as a function of the plate d. c. of the power tube) serves for the actuation of the regulator device by electrodynamic means.

Coil 2| may then be designed as a solenoid, as shown for instance, in Fig. 3. A second coil 22 is traversed by a constant D. C., and the same serves to compensate the actions of the plate feed (quiescent) current upon the regulator, or control, device. The turns thereof could be wound together with those of the solenoid 2l in a way as known from compensating windings. When lthe plate Vcurrent undergoes alterations, the iron core 23 which is attached to the arm 24 will berattracted by the coil or be repelled. In coupling relation with the magnet may be a small corrector-condenser, for example the latter being connected in parallel to the oscillator circuit or part thereof, and serving for effecting frequency corrections. The stator and the rotor plates thereof, for instance, could be made sectorshaped, pairs of the latter being disposed symmetrically to the spindle (Fig. 3). In Fig. 3 are shown the rotor sectors n and the stator sections o. The symbol o designates the outer flange ofthe stator (see Fig. 4). It will be understoodthat between the sectors o" will be spaces. In the Zero position, roughly one-half the full capacity must be present in order that the correctiveaction may cause both an increase as well as a .decrease of the oscillator frequency.

Inasmuch as no Yrestoring forces are brought to act upon the corrector device in the adsence of a regulating potential, it is generailyin the zero position (or neutral position) prior to the connection "of the automatic sharp tuning means. Hence, it is 'expedient to provide convenient restoring means. kThe later may consist, for instance, of va small iron-core 25 which is attached on the end of the arm 24 and which is drawn into a tiny coil 26 when the coil is energized.

Now, the arrangement hereinbefore outlined operates in the following fashion:

Suppose, first, that all switches have been rshifted to the position of tuning correction. YIn the absence of detuning, no control potential arises at point I8, and the normal plate current flows in the power tube. The actions thereof are just compensated by the'coil 22 so that the` regulator, or control, device remains in its position of rest. If, then, a condition of detuning arises, a positive voltage is set up, for instance at point i8; as a result the plate current of the power tube rises, the magnet is more strongly attracted into the coil 2l, the regulator means is rendered operative, and tuning is corrected. Switching over to reception, the regu- Y lator force, A'of course, disappears. It is, therefore, necessary that the regulator means, after correction of tuning has been effected and prior to switching over to regular reception of signals,

u should be locked in position by mechanical or 'electrical means (see, for instance, Fig. 4), with a View to maintaining accurate tuning conditions. Y

In order that a distinct operating knob may be made dispensable, the switching is suitably effectedgeitherfby pressing in, or withdrawing, the

arriere tuning knob, or else by the agency of a contact device which when rotating the tuning knob out of its position of rest in either direction will make or break contacts, as the case may be. The identical mechanism could be employed for severing reception during the tuning as well as for closing the contact for an electromagnetic brake or locking device .designed to lock the regulator device in position after sharp tuning, or tuning correction, has been completed. All that is necessary, tol repeat, is to take care and make arrangements so that the contact for the electrical brake will be actuated an instant before actuation of the contact designed to insure switchover for reception. To this end, it will suffice to mount these contacts in such a way one behind the other that they will be closed or opened in the proper time sequence. Similar conditions must be fulfilled in a case where purely mechanical brake means are provided.

When the tuning condenser of the receiving Yset has been coarsely set to the desired station,

the corrective condenser must be in its position of rest (neutral position) prior to the initiation of the action of automatic sharp tuning. The contact which is adapted to release the coil 26 and thus the corrector condenser, upon operation of the change-over knob for automatic sharp tuning, will be operated a certain length of time prior to braking or locking and change-over for reception.

An exemplified embodiment of an arrangement which is designed to secure the proper sequence of the actions as hereinbefore described, may be seen from Fig. 4. Referring to Fig. 4, a denotes the shaft of the operating or setting knob b in Vthe position adapted to-insure automatic tuning.

The said shaft is '-journaled and supported at c, and by the agency of a spring force d is subject to a thrust or pressure tending to force it outwardly in the receiving position (not shown). Mounted upon the said shaft,ror spindle, is a piston e disposed to shift hermetically inside a cylinder 'f attached to the support c. The said cylinder has a small air vent, and it acts like af pneumatic brake, or dashpot, tending to retard or slow up the axial shifts of the shaft. Disposed laterally to the shaft is the bank of switchover contacts g, while upon the shaft itself is a fixed drive disk h which, as shown in the drawings, serves at'the same time as a drive pulley for the rotary condenser. Actuation of the switching contacts y is insured by pressure of the disk h exerted upon the flexible lever i. In addition to the switch contacts Sii-S6 there is provided a`r contact pair S26 which is actuated by means of a flexible lever lc, and which serves for breaking the current in the restoring coil 2B (Figs. 3 and 4).

The said lever lc is so displaced in reference to the lever i that when the drive member h is moved outwardly, that is, in the direction of the arrow, k will be operated first and then only i. At the end ofthe shaft a a brake drum p is secured against rotation, while being disposed so as to be axially shiftable by the interposition of a. spring m. In axial direction there is further mounted an adjusting condenser comprising the rotor plate 1L, the stator plate o and the brake drum pv being placed opposite thereto. Operation of the rotor plate is insured by the solenoid coil 2l, inside which moves the magnet 23 which Ais connected with theshaft of the regulating condenser by the arm 24. The opposite end of the latter supports the iron core 25 which cooperates with the coil 26. The said arm 24, as

well as the rotor plate n are jointly seated upon the easily revoluble shaft 2'I.

This arrangement operates in the following manner: When the shaft a, as illustrated in Fig. 4, has been forced in, the receiver set is prepared so that sharp tuning can be accomplished automatically, the brake is off, and the corrector condenser rotor n is held in its neutral or inoperative position, since the coil 26 is energized for current fiowing therethrough. In this state, coarse tuning of the receiver set is feasible. If, then, the knob b is relaxed as soon as coarse tuning has been effected, the spindle a, by action of a. powerful spring d, will be forced outwardly, though this shifting motion is retarded by the dashpot, or pneumatic, brake e-f. Then the drive member h will rst cause operation, through the lever lc, of the contact S26, with the result that current fiowing through the coil 26 is short-circuited, and that the adjusting condenser rotor n is released, with the result that it is capable of assuming the correct regulator position in accordance with the regulator potential.

The rate of speed at which the axial shift of spindle a takes place, and the distance of the brake surface p from the regulating condenser rotor n are so chosen, that adjustment of the regulator condenser is able to take place during the time which -elapses between the release of the condenser and the instant it becomes locked in position. Immediately after the regulator condenser rotor n has been locked, the drive disk h reaches the lever i and causes switch-over to reception. If later the receiver set is desired to be changed over to another broadcast station, then the shaft must be forced inwardly. To this end, a change is first effected from reception to automatic sharp tuning (S1-Ss); next, the lock of the regulator condenser is discontinued; and, finally, by actuation of the contact S26, the condenser is returned to the neutral or inoperative position, being retained therein as long as coarse adjustment proceeds.

Another embodiment of the invention is illustrated in Fig. 5. The arrangement distinguishes itself from the one first described basically only in so far as in lieu of a simple powerstage,apush pull stage is used so that the coil comprised in the regulator device is placed symmetrically between the two power tubes of the push-pull arrangement. The compensating winding may here be dispensed with, inasmuch as the two symmetrical coils 2| and 22 of the coil may be so designed that, in neutral condition, their effects off-set. Another difference is that no differential potential is set up across the load resistances of the diodes, but that the rectified potentials at points II and I2, Fig. 5, may be directly impressed upon the grids of the power tubes. In this scheme, the ground connection is provided in the middle between the load resistances rather than at the cathode of diode 2. But, also, in this circuit organization and mechanism, as will be evident, utilization of the various tubes and cir- :uits for different purposes is insured in time sequence, after switching has been effected.

Of course, a number of other embodiments of :he basic arrangement of the invention are conzeivable. For instance, while in the above in- ;tances described by way of example no station dgnals will be audible in the course of the coarse ;uning, arrangements could also be made in such t way that the switches Si-Se, during this inerval of time, are placed on reception, and

that when relaxing this knob they are switched over so that thereupon automatic sharp tuning is rendered operative. When the knob has again been shifted out entirely, change-over to reception has been accomplished.

The circuit in Fig. 5 functions somewhat differently than in Fig. 1. The switches, except for S1 and S2, when closed permit diodes 2 and I3 to rectify the impressed signals and provide D. C. voltage across resistors 4 and I1 in opposing polarity. These voltages are applied to the grids of tubes 'I' and 'l by switches Sli-S10 when in contact with the left terminals thereof. The coils 2,I and 22 are in the space current paths of A. F. amplifiers 'I' and 'I respectively. Coils 2l and 22 function to control condenser n-o, as described in connection with Figs. 3, 4. When the voltages across 4 and I1 are equal no effect on the condenser is had. When signals are to be received switches Sz-Sa are opened; switches S9 and S10 adjusted to contact the right hand terminals; and switches S1 and S5 lare closed. In this case audio voltage is fed to the grids of audio tubes l and l through the path including condenser 5, tapped resistor 6, transformer 44, condensers 42 and 43. The diode I3 is cut out of circuit during signal demodulation.

What is claimed is:

1. In a superheterodyne receiver of the type having an intermediate frequency network, a demodulator and an audio amplifier, an intermediate frequency coupling network between said network and demodulator, an audio coupling circuit between the demodulator and the audio amplifier, a second demodulator, coupled to said intermediate coupling network, for providing demodulated output voltage in opposition to the output voltage of said first demodulator, a common tuned input circuit for said demodulators, said common circuit being included in said net- Work, means for impressing the differential of the opposed voltages on said audio amplifier, and means, responsive to the audio amplifier current, for maintaining the frequency value of the intermediate energy at an assigned value.

2. In a superheterodyne receiver of the type having an intermediate frequency network, a demodulator and an audio amplifier, an intermediate frequency coupling network between said network and demodulator, an audio coupling circuit between the demodulator and the audio amplifier, a second demodulator, coupled to said intermediate coupling network, for providing demodulated output voltage in opposition to the output voltage of said first demodulator, means for impressing the differential of the opposed voltages on said audio amplifier, and means, responsive to the audio amplifier current, for maintaining the frequency value of the intermediate energy at an assigned value, means for selectively rendering inefficient the coupling between said second demodulator and said intermediate network in order to eliminate the action of said maintaining means.

3. In a superheterodyne receiver of the type having an intermediate frequency network, a demodulator and an audio amplifier, an intermediate frequency coupling network between said network and demodulator, an audio coupling circuit between the demodulator and the audio amplifier, a second demodulator, coupled to said intermediate coupling network, for` providing demodulated output voltage in opposition to the output voltage of said first demodulator, said network including at least one tuned circuit, each demodulator being connected to said one circuit, means for impressing the differential of the opposed voltages on said audio amplifier, and means, responsive to the audio amplifier current, for maintaining the frequency value of the intermediate energy at an assigned value, and additional means for rendering said impressing means ineiective.

4. In a superheterodyne receiver of the type including an intermediate frequency network comprising at least one resonant circuit tuned to the operating intermediatee frequency, a iirst detector connected to said circuit, an audio amplifier circuit connected to said detector, a second detector connected to said one circuit, means for deriving from the two detectors a direct current voltage which represents the differential of the output voltages of said dectors, said one circuit being so connected and arranged with respect to said two detectors that said derived voltage has a value which varies with the frequency shift of the intermediate energy, means impressing said derived voltage on said amplier to permit the latter to amplify the derived voltage, and mistuning correction means responsive to the amplied derived voltage for adjusting the receiver sufficiently to compensate for saidlfrequency shift.

5. In a superheterodyne receiver of the type including an intermediate frequency network comprising at least one resonant circuit tuned to the operating intermediate frequency, a first detector connected to said circuit, an audio amplifier circuit connected to said detector, a second detector connected to said one circuit, means for deriving from the two detectors a direct current voltage which represents the differential of the output voltages of said detectors, said one circuit being so connected and arranged with t respect to said two detectors that said derived voltage has a value which varies with the frequency shift of the intermediate energy, means impressing said derived Voltage on said amplifier to permit the latter to amplify the derived voltage, mistuning correction means responsive to the amplified derived voltage for adjusting the receiver sufficiently to compensate for said frequency shift, and switch means for selectively rendering ineffective the electrical association between the said one circuit and said second detector thereby to eliminate the action of said correction means without affecting the connection between the first detector and audio amplifier.

WOLFGANG F. EWALD.

KARL WILHELM.

HELMUT PITSCI-I.

OTTO TXEN. 

